The present invention is directed to semiconductor devices and, more particularly, to the fabrication and arrangement of deep trench structures used in semiconductor devices.
Present methods of fabricating the deep trench structures of a semiconductor device are often costly and require process steps that are hard to control. As an example of a deep trench fabrication process, a pad oxide layer and a pad nitride layer are first deposited atop a silicon substrate, and then a hard mask layer is deposited atop the nitride layer. The hard mask layer and the pad nitride layer are then patterned and etched using a lithographic step, and the hard mask layer is then used to mask the etching of a deep trench.
Next, the hard mask layer is removed, and a doped glass layer is deposited along the walls and bottom of the trench as well as atop the nitride layer. A further step is then carried out to pattern and remove the doped glass from atop the nitride layer and from the walls of the upper portion of the trench. An oxide cap is then deposited over the remaining portion of the doped glass, as well as over the walls of the rest of the trench and atop the nitride layer, and an anneal step is carried out to drive dopants from the doped glass into the silicon substrate and form a buried plate. The oxide cap and the doped glass are then removed, and a thin dielectric layer is deposited along the sides of the trench.
The lower portion of the trench is filled with polysilicon to form the node conductor. The top surface of the device is then planarized to remove any portion of the polysilicon that is atop the nitride layer, and the polysilicon is recessed to the intended depth of the collar. The dielectric film is removed from the exposed upper portion of the trench, and the trench collar oxide layer is then deposited and directionally etched back to remove any portion of the trench collar oxide layer that is atop the nitride layer and on top of the node polysilicon. The remainder of the polysilicon layer is next deposited, and the device is again planarized to remove any polysilicon that is atop the nitride layer. The nitride layer protects the surrounding silicon during the polysilicon etch step.
The trench collar oxide is then recessed back preferably using a wet etch step. The oxide recess forms a divot at a location where the collar oxide is removed below the level of polysilicon fill. The divot is then filled by again filling the trench with polysilicon and then recessing the polysilicon to a desired level. The polysilicon region is subsequently doped in high temperature processing steps by the prior deposited polysilicon, and the dopant subsequently out-diffuses into the substrate to form a buried strap region.
As semiconductor devices become increasingly smaller, the aspect ratio, namely the ratio of the height of the trench to the width of the trench, increases making such known processes harder to control. Further, it is also desirable to form trenches having shapes other than the structure fabricated by the known process and to increase the capacitors of the device.
It is therefore desirable to provide a process for fabricating a deep trench structure of a semiconductor device and the arrangement of such a structure that overcomes the above problems.